The invention relates to hybrid amplifiers for optical communication systems. More particularly, the invention relates to minimizing the noise figure of a hybrid amplifier that hybridizes Raman and rare-earth doped amplifier components.
Hybrid amplifiers are conventional elements in modern optical communication systems. A hybrid amplifier may combine or hybridize a rare-earth doped fiber amplifier such as an EDFA (erbium doped fiber amplifier) and a distributed amplifier such as a Raman amplifier. Such hybrid amplifiers are typically designed for optimum performance at a specific fiber or span loss (e.g. designed for 20 or 25 dB loss).
As is also known in the art, the maximum distance an optical signal can travel on a segment (span) of optical fiber depends, among other things, on the noise performance of the amplifier boosting the signal. A noisy amplifier can bury the signal (reduce the signal-to-noise ratio) or otherwise make the signal indistinguishable from the noise particularly as the signal is attenuated across the span.
The noise performance may be quantified as a noise figure which is expressed in dB. An amplifier with a lower noise figure translates into a longer reach for the optical communications system.
Extending the reach or maximum transmission distance at which the signal can be reliably recovered is an important goal in optical communication systems. Therefore, a need exists for a hybrid amplifier with a minimized noise figure.
The invention is a strategy that combines Raman and EDFA amplifiers in such as way as to minimize the amplifier noise figure for the total hybrid amp. By minimizing the noise figure, the invention also permits an optical signal to be transmitted a longer distance and still have a sufficient signal-to-noise ratio so as to be recoverable.
The invention achieves this goal by determining a gain balance between the Raman amplifier and the EDFA that minimizes the noise figure for a particular span loss. In other words, the invention determines how much gain should be applied by the Raman amplifier and how much gain should be applied by the EDFA that would generate the minimum amount of Amplified Spontaneous Emission (ASE) noise for the hybrid amplifier.
A hybrid amplifier according to the invention includes a rare-earth doped fiber amplifier operatively connected to a span of optical fiber; a Raman pump operatively connected to the span of optical fiber and operating at a wavelength selected to provide Raman gain to an optical signal travelling in the span of optical fiber; and a controller operatively connected to the rare-earth doped fiber amplifier and said Raman pump; the controller controlling the Raman pump to provide a Raman gain value that minimizes a noise figure of the hybrid amplifier for a span loss of the span of optical fiber to which the Raman pump and the rare-earth doped fiber amplifier are operatively connected; and the controller controlling the rare-earth doped fiber amplifier to amplify the optical signal in accordance with the Raman gain value.
Furthermore, the controller may control the rare-earth doped fiber amplifier to amplify the optical signal at a gain value related to the difference between the span loss and the Raman gain value.
Still further, the rare-earth doped fiber amplifier may be an EDFA.
The invention also includes a method of minimizing noise generated by a hybrid amplifier having a rare-earth doped fiber amplifier and a Raman amplifier, including: generating a plurality of functions, each function relating a noise figure of the hybrid amplifier and a Raman gain of the Raman amplifier for a particular span loss; selecting a span loss value from among the particular span losses; determining a Raman gain value that minimizes the noise figure of the hybrid amplifier from the function corresponding to the selected span loss value; controlling the Raman amplifier to amplify an optical signal at the determined Raman gain value; and controlling the rare-earth doped fiber amplifier to amplify the optical signal in accordance with the Raman gain value.
If only one span loss is contemplated, then the invention may include a method of minimizing noise generated by a hybrid amplifier having a rare-earth doped fiber amplifier and a Raman amplifier, including: generating a function relating a noise figure of the hybrid amplifier and a Raman gain of the Raman amplifier for a span loss of the span of optical fiber to which the Raman amplifier and the rare-earth doped fiber amplifier are operatively connected; determining a Raman gain value that minimizes the noise figure of the hybrid amplifier from the generated function; controlling the Raman amplifier to amplify an optical signal at the determined Raman gain value; and controlling the rare-earth doped fiber amplifier to amplify the optical signal in accordance with the Raman gain value.
The invention may also include setting the rare-earth doped fiber amplifier gain to a value related to the difference between the selected (or single) span loss value and the determined Raman gain value.
A hybrid amplifier according to the invention may include a rare-earth doped fiber amplifier such as an EDFA operatively connected to a span of optical fiber; a Raman pump operatively connected to the span of optical fiber and operating at a wavelength selected to provide Raman gain to an optical signal travelling in the span of optical fiber; a controller operatively connected to the rare-earth doped fiber amplifier and the Raman pump; a memory device storing a Raman gain value and a rare-earth doped amplifier gain value that minimizes a noise figure of the hybrid amplifier for a span loss of the span of optical fiber to which the Raman pump and the rare-earth doped fiber amplifier are operatively connected; the controller controlling the Raman pump to provide Raman gain substantially equal to the Raman gain value stored by the memory device; and the controller controlling the rare-earth doped fiber amplifier to provide gain substantially equal to the rare-earth doped amplifier gain value stored by the memory device.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.